In recent years, an organic electroluminescence element using an organic substance (hereinafter, appropriately abbreviated as “organic EL element” or “OLED; Organic Light-Emitting Diode”.) is promised as a use as a solid light emitting large-area full color display element that is lightweight, thin, high-efficient and inexpensive and a light source array, and the research and development is actively advanced.
Particularly in a mobile body (cell phone, automobile, aircraft), a lighting that is thinner and lighter than conventional ones and hard to crack (lighting composed of a flexible substrate) is expected. Also, while these new values are expected, performance to an existing fluorescent light and white LED is currently low, and technology for further higher efficiency and prolonged lifetime are required.
An organic EL element is a thin-film all solid element constituting an organic function layer (single layer part or multilayer part) containing an organic light emitting material having a thickness of about 0.1 μm, between a pair of anode and cathode formed on the film. When a relatively low voltage of about 2 to 20 V is applied to such organic EL element, electrons are injected into an organic compound layer from a cathode, and holes are injected from an anode. It is known that these electrons and holes are recombined in a light emitting layer, and when an energy level returns to a valence band from a conduction band, energy is released as light, thereby obtaining light emission, and it is a technology expected as a next-generation flat display or lighting.
Furthermore, in the organic EL element utilizing phosphorescence emission recently found, about four times light emission efficiency, as compared to former one utilizing fluorescence emission, can be realized in principle. Thus, including the development of the materials, research and development of the layer constitution of the organic function layer and the electrodes are advanced throughout the world. Particularly, as one of the measures to prevent global warming, an application to a lighting apparatus which predominates in human energy consumption has been started to study, and toward a practical use of a white light emitting panel that can replace conventional lighting apparatus, performance improvement and an attempt to reduce costs are actively carried out.
However, a phosphorescent organic EL element (OLED; Organic Light-Emitting Diode) is sensitive to oxygen and moisture, thus when being used as the flexible lighting as described above, an expensive barrier film having high barrier properties and treatment of step with low productivity (vacuum deposition, application process in an inert atmosphere, and the like) are required, thus it cannot be necessarily said as inexpensive and highly producible.
On the other hand, as an all solid electric field electroluminescence element in which high efficiency as same as the phosphorescence organic EL element (OLED) is recently expected, a quantum dot LED (QLED) is suggested (Patent Document 1). In QLED, the excited singlet level (S1) and the excited triplet level (T1) are close each other and thus an intersystem crossing between S1 and T1 is thermally possible. Therefore, while it is substantially fluorescence (emission from S1), a quantum yield at lower than 100% can be expected.
Here, it is an emission from fluorescence, thus is hard to be deactivated by an oxygen molecule that is a molecule of a base triplet, and as disclosed in Patent Document 2, it tends to be strong also against moisture, thus it is supposed that the emission can be produced in a highly producible environment such as under the atmosphere. Furthermore, the QLED has a reverse layer constitution as in Patent Document 3, whereby an alkali metal halide such as lithium fluoride that is likely to be deteriorated by oxygen or moisture is not contained in the layer, thus it is expected that more stable element is obtained.
In addition, in the reverse layer constitution, a hole block layer and an electron transport layer such as a metal oxide having a level close to HOMO/LUMO of quantum dot having a very deep level and an electron transport layer can be used, thus a high efficiency can be expected.
However, in Patent Document 3, a zinc oxide layer is formed by calcining at about 300° C. for 5 minutes on the first electrode (transparent electrode, ITO) on the substrate. In such application process, an electroluminescence element on the plastic substrate cannot be produced by the simple application under atmospheric pressure as described above, and it has been a barrier to obtain an ideal electroluminescence element. In addition, a metal oxide thin film generally has high surface roughness, and a dark spot caused by a leak is likely to occur, and has a problem also in the lifetime.
In addition, in Patent Documents 1 to 3 described above, a light emitting layer composed of a single quantum dot layer is used. However, when quantum dots aggregate, the light emission efficiency is degraded, thus it is effective to properly disperse the quantum dots in the host for the improvement in light emission efficiency. Yet, an electric field electroluminescence element using quantum dots, particularly, a host compound appropriate as a blue electroluminescence element has not been found so far, and still has a problem with regard to the lifetime.